1. Field of the Invention
The present invention relates to the field of photographic equipment used to produce a strong, even, diffused source of light. More particularly, the present invention relates to photographic equipment used to duplicate film wherein the master copy, which is being duplicated, is illuminated by a diffused source of light.
2. Description of the Prior Art
The abrasion or other scratching or master film copies presents a particularly acute problem in the film printing. In order to produce the highest possible illumination at the aperture of the printer head, most printers incorporate a specular or collimated illumination and imaging system which is produced by a system of mirrors and condenser lenses to collect and guide the light from a bright lamp to a projector film gate. The light so produced is characterized by substantially parallel rays which tend to accentuate scratches in the master copy. Most shallow scratches occur on the film support base, but whether on the base or in the emulsion, the effect of the scratch is to scatter collimated light which passes through the film. The scratches and other surface imperfections in the master copy scatter the light and thereby cause a shadow or line to be visible in the projected image. It may be appreciated that the higher the degree of collimation, (which is characteristic of most high intensity light sources) the greater is the accentuation of the surface imperfections of the projected image.
Three techniques are currently employed for the removal of both emulsion and film base scratches. Scratches may be partially or completely removed through "liquid-gate" optical printing. The process requires that the scratched master copy be momentarily immersed or covered with a transparent fluid during the period when it passes into contact with a duplicating stock or through the intermittent of the optical printer's projector. A fluid is employed which has substantially the same refractive index as the film base or the emulsion. The fluid fills in the scratched valleys and allows the light from the lamp house and condensers to pass straight through the film without being displaced through the scratches. The use of a "liquid-gate" is generally an expensive and complex process. Such a process requires the use of very elaborate machinery which includes pumps, filters, bubble traps, heaters, a vacuum system and electronic controls.
The second technique simply involves enlarging those portions of the master copy which are unscratched. The usefulness of this technique is limited to those instances in which the surface imperfection is systematically confined to an edge portion of the master copy so that the selective enlargement of the master copy is not so great as to sacrifice large amounts of visual information, to unbalance the composition of the original scent, or to increase the grain size of the image.
The third technique to minimize scratches involves the use of a diffused light source to illuminate the master copy. If the illuminating light is thoroughly diffused, the perturbations caused by scratches in the master copy will not create a visual perturbation in the projected image, since the diffuse light incident on the film will have substantially the same degree of diffuseness after transmission through the film. The drawback of prior art optical printers, using a diffused light system, is that such diffusers lowered the illumination level considerably. Moreover, none of these diffusers are able to deliver a strong, even source of diffuse light at any significiant distance from the diffuser. The lower illumination level requires that the optical printer operate at a slower and less economical printer speed. With the prior art diffusers it is not uncommon that the printer speed will be eight times slower (illumination level will be three stops lower) than speeds obtainable with the liquid gate process.
However, where a diffuse light has been previously used, a polished or abraded flat glass plate or opal glass plate is typically placed immediately behind the film gate so that substantially all the light transmitted through the plate is incident upon the film gate. However, as the distance between the diffusing plate and the film gate increases, the light intensity decreases inversely as the square of the distance and the measure of diffusion of the light likewise dramatically decreases. Therefore, in the prior art, it is only possible to use diffusing plates when positioned close to the film. This has the disadvantage that the diffusing plate, typically ground or opal glass, has a certain grain or texture defined by its mode of manufacture, which tends to be imposed upon the projected image. Moreover, it is the observed fact that when the diffusing optics is positioned substantially adjacent to the film gate, there is a tendency for grease, oil, and dirt to accumulate on the diffusing optics. Also, since the diffusing opal glass is adjacent to the film plane, it is imperative that the light is evenly distributed before it strikes the diffusing glass.
Therefore, what is needed is a simple, inexpensive means for illuminating a master copy of film in such a manner that the existence of scratches and other surface imperfections may be de-emphasized. In addition such illumination must be accomplished without incurring the disadvantages of loss of light intensity, short range of illumination, uneven illumination field, low measure of diffuseness, or tendency to foul which characterizes prior art diffusion systems or the disadvantages of undue complex and expense which characterizes "liquid-gate" systems.